That's because you don't get big biceps from doing curls, you get them from moving weight with compound exercises. Deads, squats, rows, pullups, dips..that's how you get big and, in turn, get big biceps.
This is debatable to a few but I don't see any guys with great arms over the last 4 or more decades who got big arms without moving serious weight on direct, non-compounds like curls (schwarzenegger, bertil fox), preachers (Scott), cable curls (Coleman) and triceps extensions (oliva, coleman, scott, etc). Even Mariusz, world's strongest man with great arms, does direct arm work, unlike other world's strongest man contestants, and you can see the difference.
Don't worry about the number of exercises done for each muscle-decide on a total number of sets to be done for each muscle, then divide up the sets by the number of exercises done.
Shouldn't be a problem coming up with a 2-day split, there are all sorts of variations-push/pull, upper body/lower+abs, heavy torso muscles/lower body, etc.